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    <title>Document</title>
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<script>
    alert("hello!wold" ) 
    console.log('succed' )
    //变量
    let num1=123,num2= 'hello' 
    console.log(num1) 
    console.log(num2)
    console.log(typeof(num1)) //打印数据类型
    console.log(typeof(num2)) 
    console.log('---------------------------------------')
    let flag=true
    console.log(flag)
    console.log(typeof(flag)) 
    flag=false
    console.log(flag)
    console.log(typeof(flag)) 
    console.log('---------------------------------------')
    let str1='子曰:“学而时习之” '
    console.log(str1)
    console.log(typeof(str1))
    // 子曰:‘学而时习之’
    //he said:'day day study'
    //在字符型中，单引号中用单引号，或双引号中用双引号，需要利用转义符号“\” 
    let str2 = 'he said:\'day day study\''
     console.log(str2)
    console.log(typeof(str2))
     str2="子曰: \"学而时习之\”"
      console.log(str2)
    console.log(typeof(str2)) 
    console.log('---------------------------------------')
    let NULL=null
     console.log(NULL)
    console.log(typeof(NULL)) 
    console.log('---------------------------------------')
    let unNum
    console.log(unNum)
    console.log(typeof(unNum)) 
    let str3 =''
    console.log(str3)
    console.log(typeof(str3))
</script>
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